Two new "superheavy" elements have been made by bombarding lead atoms with energy-packed krypton atoms at the rate of two trillion per second.

After 11 days, the scientists working at the Lawrence Berkeley National Laboratory, US, had produced just three atoms of element 118. These each contained 118 protons and 175 neutrons in their nucleii.

The new atoms decayed almost instantly to element 116, which itself was short-lived. But, for that brief moment, they were the only three atoms of these elements ever to have existed on Earth.

Preparations for the atom-crashing experiment

Ken Gregorich, the nuclear chemist who led the discovery team, said: "Our unexpected success in producing these superheavy elements opens up a whole world of possibilities using similar reactions: new elements and isotopes."

US Secretary of Energy, Bill Richardson, commented: "This stunning discovery opens the door to further insights into the structure of the atomic nucleus."

Unstable combination

Atoms consist of a central nucleus surrounded by a cloud of electrons. The nucleus consists of protons and neutrons.

But not all combinations of neutrons and protons are stable. In nature, no element heavier than uranium, with 92 protons and 146 neutrons, can normally be found.

Scientists can make heavier ones by colliding two large nuclei together and hoping that they will form a new, heavier nucleus for a short time.

The cyclotron in which the new elements were created

One of the most significant aspects of the new elements is that their decay sequence is consistent with theories that predict an "island of stability" for atoms containing approximately 114 protons and 184 neutrons.

"We jumped over a sea of instability on to an island of stability that theories have been predicting since the 1970s," said nuclear physicist Victor Ninov. He is the first author of a paper on the discovery submitted to Physical Review Letters journal.

Atomic structure

Synthetic elements are often short-lived, but provide scientists with valuable insights into the structure of atomic nuclei. They also offer opportunities to study the chemical properties of the elements heavier than uranium.

I-Yang Lee, scientific director of the atom smasher at Lawrence Berkeley National Laboratory, said "From the discovery of these two new superheavy elements, it is now clear that the island of stability can be reached.

"Additionally, similar reactions can be used to produce other elements and isotopes, providing a rich new region for the study of nuclear properties."

Element 118 takes less than a thousandth of a second to decay by emitting an alpha particle. This leaves behind an isotope of element 116 which contains 116 protons and 173 neutrons.

This daughter is also radioactive, alpha-decaying to an isotope of element 114.

The chain of successive alpha decays continues until at least element 106.